Current-day front-end modules, FEM, in mobile communication devices, such as cellular handsets operating according to the long-term evolution, LTE, standard for data and voice communications rely heavily on the use of frequency-division duplexing, FDD, meaning that the transmitter and receiver operate concurrently, at a frequency offset. More and more bands are defined for FDD, and consequently the amount of filters that need to be implemented in the FEM increases. For each band, typically one filter is provided in the transmit, TX, path towards the antenna (preventing TX noise at the receive, RX, frequency to leak into the RX) and one filter is provided in the RX path between antenna and RX (preventing the TX signal to leak into the RX). Currently, fixed-frequency filters are used, which have the disadvantage of being costly, bulky and which do not scale up when more bands are required.
Electrical-balance duplexers, EBDs, of which an example implementation is shown in FIG. 1, promise to replace numerous surface acoustic wave, SAW, based filters in the front-end module of cellular handsets. Instead of supporting only one band, in this case a single duplexer is tuned to support multiple bands. They operate based on signal cancellation using a hybrid transformer, essentially by ‘balancing’ an antenna ZANT and a tunable impedance circuit, the so-called balance network or ZBAL.
A disadvantage of this architecture is the bandwidth for which sufficient balance or isolation can be achieved, i.e., matching ZBAL to the antenna impedance ZANT, for real antennas that have a highly frequency-dependent impedance characteristic. In FDD operation, isolation is needed at both a first frequency, being the TX frequency, and a second frequency, being the RX frequency. At the TX frequency, the TX signal itself is too large for the receiver to sustain so its power level needs to be reduced through isolation, and at the RX frequency the transmitter emits a large amount of noise that needs to be removed to avoid reducing the signal-to-noise-ratio, SNR, for the typical (small) wanted signal levels that arrive at the antenna. In the EBD, it is possible to isolate one of the two frequencies, but not the other due to limited isolation bandwidth. This is caused by the frequency dependency of the impedance of typical commercial antennas.